Audio reproduction apparatus, feedback system and method

ABSTRACT

The audio reproduction apparatus comprises a cost input for inputting a mathematical cost derived from a measurement, which measurement is user-influenceable and a conditioning unit, capable of delivering an output audio signal in dependence of the mathematical cost, characterized in that the conditioning unit comprises an audio processing means arranged to process an input audio signal to derive the output audio signal with a reproduction quality in dependence of the mathematical cost. As a reproduction quality the position of a virtual sound source and the quality of a stereo signal are also possible. A system comprising the audio reproduction apparatus, a measurement device and a sound production device and a method of to deriving the output audio signal with a reproduction quality in dependence of the mathematical cost are also presented.

The invention relates to an audio reproduction apparatus comprising:

input means for inputting an input audio signal;

an output for outputting an output audio signal derived from the inputaudio signal;

a cost input for inputting a mathematical cost derived from ameasurement, which measurement is user-influenceable; and

a conditioning unit, capable of delivering the output audio signal independence of the mathematical cost.

The invention also relates to an audio feedback system comprising:

an audio source;

a measurement device arranged to deliver a measurement which isuser-influenceable;

a mathematical cost calculation unit, arranged to derive a mathematicalcost from the measurement;

a sound production device; and

a conditioning unit arranged to receive an input audio signal from theaudio source, to receive the mathematical cost, and to deliver to thesound production device an output audio signal derived from the inputaudio signal, in dependence of the mathematical cost.

The invention also relates to a method of deriving an output audiosignal from an input audio signal in dependence of a mathematical costderived from a measurement which is user-influenceable.

The invention also relates to a computer program for execution by aprocessor, describing above mentioned method.

The invention also relates to a data carrier storing the computerprogram.

An embodiment of such an audio reproduction apparatus is known from U.S.Pat. No. 4,788,983. The known apparatus is designed for use by a personperforming a sports activity, who wants to listen to music. The knownapparatus contains a conditioning unit capable of transmitting an inputaudio signal from a walkman as an output audio signal to headphones. Theconditioning unit also receives a mathematical cost signal from a heartrate measurement device. A user specifies according to e.g. his age orsex a safe window of heart rate which he wants to use during histraining. If his heart rate is too low, he is clearly not getting enoughexercise. On the other hand, if his heart rate is too high, his exercisemight be unhealthy. The conditioning unit transmits the input audiosignal only if the measured heart rate is within the desired window, andotherwise no sound is sent to the headphones.

It is a disadvantage of the known apparatus that such a crude regulationof the output audio signal is not user friendly. E.g. if the window isnarrow, it is difficult for a user to judge whether he has lost hismusic because he is running too slow or too fast.

It is an object of the invention to provide an audio reproductionapparatus of the kind described in the opening paragraph, which isrelatively versatile as far as the regulation of the output signal isconcerned.

The object is realized in that the conditioning unit comprises an audioprocessing means arranged to process the input audio signal to derivethe output audio signal with a reproduction quality in dependence of themathematical cost. The conditioning unit of the known audio reproductionapparatus contains elements which only implement a switch function. Incase the heart rate is outside the window, no signal is sent to theheadphones. This is not very desirable. If the user is training just alittle too soft, he will hear absolutely no sound. Rather than to bemotivating to start running harder again, this can be very demotivatingfor certain users. It is desirable that there is a gradual change, sothat the user can underachieve during a certain period and still havemusic. Furthermore heart rate measurements are not always reliable, e.g.if a signal of a nearby second user is picked up. The user is thenpunished for something out of his control. The audio reproductionapparatus according to the present invention is arranged to offer theuser many more versatile strategies of responding to his sportsactivity, embodied as strategies of calculating the mathematical cost.The apparatus according to the invention is also arranged to providemore versatile output audio presentation strategies. Instead of justswitching off the output audio signal, the apparatus of the inventionoffers options of changing the reproduction quality of the output audiosignal. This is a physical measurable and determinable correlate to theperceptible psycho-acoustical quality of the sound. E.g., the audioreproduction apparatus may be able to gradually lower the soundamplitude, leading to less intelligible music. Alternatively, if stereois present an underachieving user can be punished in that the audioreproduction apparatus delivers mono instead of stereo, in which casethe number of independent output signals is a reproduction qualitymeasure of the perceived psycho-acoustical quality.

In an embodiment, the reproduction quality comprises a three-dimensionalposition of a virtual sound source, the audio processing means beingable to simulate the virtual sound source by means of the output audiosignal. Of the set of all audio-processing functions which can beapplied to obtain some psycho-acoustical quality, some realize the audiopositioning of a virtual sound source in three-dimensional space aroundthe head of a user. This is especially interesting for the usertraveling a distance—real or virtual—such as a jogger or somebody on abicycle hometrainer. If he instructs the apparatus that he should run ata certain speed, he should be at a certain position—real or virtual—at acertain time. The audio reproduction apparatus can position the virtualsound source by sending an appropriate audio signal to a left and aright loudspeaker of the headphones, the virtual sound source being e.g.two virtual loudspeakers at a distance of 1 meters in front of the user.If the user runs too slow the virtual loudspeakers move away from him,and this can be simulated if desired by making the sound emerging fromthe virtual loudspeakers become ever less audible. The user can catch upwith the loudspeakers by running faster. By adding syntheticreverberations other three-dimensional audio position qualitymeasurements can be influenced, such as the illusion of a wall in frontof the user.

In a modification of the embodiment, the audio processing means comprisea filter arranged to simulate the position of virtual sound source byderiving the output audio signal by filtering the input audio signalwith a user dependent head related transfer function (HRTF). By means ofan HRTF a sound source such as a virtual loudspeaker can be accuratelypositioned. The input audio signal for the left resp. the rightheadphone loudspeaker is filtered by a respective HRTF, simulating thepath through a virtual room of sound from an actual loudspeaker at aposition in the room to the respective ear of the user.

In another modification of the embodiment or a further variation on theforegoing modification, the audio processing means comprise an audioprocessing unit arranged to simulate the position of the virtual soundsource by changing a property of the output audio signal selected fromsignal amplitude and added reverberation. Both properties are simplesignal processing functions to realize the illusion of sound with aspecific three-dimensional audio position quality.

The audio processing means can also be arranged to derive a secondoutput audio signal, together with the output audio signal constitutinga stereo audio signal, the audio processing means being arranged toderive the stereo audio signal from the input audio signal with aspecified stereo quality dependent on the mathematical cost. Signalprocessing functions influencing the stereo quality are e.g. thefollowing:

the virtual loudspeakers are moved closer to each other if the userunderachieves;

the audio signals for the left and right virtual loudspeakers are mademore similar if the user underachieves; or

one of the virtual loudspeakers disappears if the user underachieves.

The last option can also be implemented as a—gradual or brusque—switchbetween multichannel surround such as Dolby 5.1 and 2 channel stereo.Varying the stereo quality is advantageous given the widespread presenceof stereo sound.

It is advantageous if the reproduction quality comprises a specificationof the distribution of frequencies of the output audio signal. Bychanging the frequency content of the output audio signal, the audioprocessing means can simulate other effects. E.g. an underachiever ispunished by removing the bass of the audio signal. The three-dimensionalposition of a virtual sound source can be influenced by such an audioprocessing function too. E.g. the audio processing means may be arrangedto remove high frequencies as if the sound has to travel a long waythrough a thick fog, or resides at a depth in a virtual wall.

It is also advantageous if the audio reproduction apparatus comprises afirst quality calculation unit for determining the reproduction qualityfor use in the subsequent derivation of the output audio signal by theaudio processing means. The reproduction quality is a property of theoutput audio signal. The audio reproduction apparatus could measure iton the output audio signal, but then the input audio signal first has tobe processed by an a priori unknown processing algorithm. A correlatereproduction quality can be determined by the first quality calculationunit and send to the audio processing means which applies acorresponding processing function. E.g. changing the angle of twovirtual loudspeakers has an influence on the stereo quality, and it isin general not necessary to specify what numerical stereo quality a userexperiences. If more accuracy is desired, a specific angle function canbe stored in a memory, e.g. based upon user panel tests. It is alsopossible that the user specifies a relation between e.g. his runningspeed and the angle himself, or chooses between a number of prestoredfunctions, some of which change the angle slowly and other quickly.

Alternatively or additionally it is advantageous if the audioreproduction apparatus comprises quality measuring means for measuringan output quality measure of the output audio signal, and comprisesparameter value calculation means for calculating a parameter value, foruse in the subsequent derivation of the output audio signal by the audioprocessing means. If the quality of the output signal is measured, itcan be fed back for changing the processing on the input audio signalfor future times. Such a feedback control loop obtains the desiredreproduction quality after some tuning time. The parameter valuecalculation means can take an incompatibility between the result of thereproduction quality measurement and the desired quality measurementinto account. The parameter value is changed accordingly, steering theprocessing function until it obtains the desired audio output signalreproduction quality.

In another embodiment a mathematical cost calculation unit is comprisedwhich is arranged to derive the mathematical cost from the measurementreceivable from a measurement device. A mathematical cost can be derivedfrom any kind of apparatus, e.g. a random generator if the audioreproduction apparatus is used in a competitive game. Typically themathematical cost may be determined on the basis of a measurement whichthe user can influence, such as his running speed, heart rate, etc. Ameasurement device could send the mathematical cost directly to theapparatus, e.g. as a coded number. Typically however the audioreproduction apparatus may contain the new functionality, so that it canbe used with an off-the-shelf measurement device.

In a modification of the previous embodiment, the mathematical costcalculation unit is arranged to derive the mathematical cost based on adifference between the measurement and a chosen value. E.g., the usersets this chosen value being his desired running speed as 10 km/h, orhis desired heart rate as 180 bpm. The quality is then e.g. determinedas the actual running speed minus 10 km/h. The faster he runs, the morethe reproduction quality changes. Or in an alternative version, if heruns a little harder nothing happens, and if he runs a lot harder theaudio processing means are arranged so that the reproduction qualitystarts changing gradually depending on the amount of time he is runningharder than the chosen value.

In an alternative modification of the previous embodiment or in additionto the previous modification, the mathematical cost calculation unit isarranged to derive the mathematical cost from a biometric measurement.Engineering quality audio systems and biometric measurements are totallyunrelated fields of technology. Apparently, nobody sees a need ofcombining them. Biometric measurement systems are usually designed byengineers who work in close cooperation with physicians, and prioritiesin this field are exactness and robustness of the measurements andsafety. The quality of audio reproduction is more an artistic matter oftaste. This has led to the fact that biometric measurements are usuallydisplayed on numeric displays. An exception is the beeps of a medicalmonitor, but the audio functionality of such monitors is designed forsimplicity rather than artistic reproduction quality. There is a needfor user-friendly feedback of biometric data while training, since auser does not like to watch a display continuously while training. Soundhowever when present automatically enters the user's ear.

The audio feedback system is characterized in that the conditioning unitcomprises an audio processing means arranged to process the input audiosignal for the derivation of the output audio signal with a reproductionquality in dependence of the mathematical cost. It is advantageous toproduce the system as a whole since then all components can be realizedas being optimally tuned to each other.

The method of deriving an output audio signal from the input audiosignal in dependence of the mathematical cost derived from a measurementwhich is user-influenceable is characterized in that the output signalis derived with a specified reproduction quality dependent on themathematical cost.

These and other aspects of the audio reproduction apparatus, the method,the audio feedback system, the computer program and the data carrieraccording to the invention will be apparent from and elucidated withreference to the implementations and embodiments described hereinafter,and with reference to the accompanying drawings, which serve merely asnon limiting illustrations.

In the drawings:

FIG. 1 schematically shows an application of the audio reproductionapparatus;

FIG. 2 schematically shows an embodiment of the audio reproductionapparatus;

FIG. 3 schematically shows an embodiment of the audio processing meansof the audio reproduction apparatus;

FIG. 4 schematically shows an embodiment of the audio feedback system;

FIG. 5 schematically shows an embodiment of the data carrier;

FIG. 6 a and b schematically show a respective example of a mathematicalcost function;

FIG. 7 schematically shows an example specification of a positionquality as a function of mathematical cost; and

FIG. 8 schematically shows an example of a frequency spectrum of anoutput audio signal output by the audio reproduction apparatus.

In these Figures elements drawn dashed are virtual in FIG. 1, andoptional in the other Figures, depending on the desired embodiment. Notall elements present in the illustrative embodiment of the audioprocessing means in FIG. 3 need be present in an alternative embodiment.

FIG. 1 shows a user 100 of the audio reproduction apparatus 200according to the invention, who is jogging. He could e.g. also be rowingon an indoors rowing machine. While doing his sports activity he islistening to music coming as an output audio signal o—see FIG. 2—fromthe audio reproduction apparatus 200, being e.g. a portable audio playersuch as an MP3 player, and reproduced by the left and right headphoneloudspeaker 114 and 115 of a sound production device 102, typicallybeing headphones. The reproduction quality R of the music is changed bythe audio reproduction apparatus 200 in dependence of the sportsperformance of the user 100, e.g. varying between Dolby 5.1 and monowithout bass frequencies. E.g. if he runs too slow, he is penalized withmusic of bad reproduction quality R. His performance can be measured byat least one of various sensors. E.g. a pace meter 108 connected to hissports shoe or another measurement device like a heart rate meter 106delivers a measurement signal m. For the heart rate meter 106 thismeasurement can be e.g. a PQRST complex of an electrocardiogram, a timesequence of pulses, or a number representing the heart rate. The audioreproduction apparatus 200 converts this measurement m to a reproductionquality R.

In a simple variant of the audio reproduction apparatus 200, the audioreproduction strategy is fixed, and the user 100 can only specify theway in which the measurement is transformed into a mathematical cost c.Note that for clarity reasons the examples are described for anembodiment in which all the mathematical transformations are realized assoftware algorithms running on a processor, but dedicated hardwarecircuitry could also be used. E.g. the user 100 can specify an intervaliv in which his heart rate should lie such as [LL, LU] in FIG. 6 a. Amathematical cost function 602 is shown in a coordinate system 600 withon the x-axis the heart rate measurement m minus a chosen value d and onthe y-axis the mathematical cost c. This chosen value d is set by theuser as the target heart rate for his training, e.g. 180 bpm. Theinterval [LL, LU] can be symmetrical or asymmetrical around the chosenvalue d. The mathematical cost function can be fixed in the hardware ofthe audio reproduction apparatus 200 of FIG. 2, or the user can specifyby means of user interface means 311 of FIG. 3 how the mathematical costchanges with m-d. E.g. as in FIG. 6 a up to the marker values ML and MUthe mathematical cost changes linearly with a small slope, while betweenthe marker values and the limits of the interval iv the mathematicalcost slopes more steeply, and outside the interval iv the mathematicalcost increases very steeply. In an alternative mathematical costfunction 606 specification shown in FIG. 6 b, the mathematical cost isnon-zero and linearly changing only outside the training interval iv.E.g. in FIG. 6 b the user 100 has designed a cost function with negativevalues if he is running too slow. A negative cost can then duringreproduction easily be mapped e.g. as a negative angle α of a virtualsound source, and a positive cost corresponding to speeds which are toohigh as a positive angle α. In this way both cases can be easilydiscriminated. The user has freedom in designing the cost function bychoosing e.g. training interval limits, quickness of change of thecost—which can be translated into quickness of change of audioreproduction quality R—, whether only measurements below the chosenvalue d are leading to nonzero cost, etc.

The user interface means 311 are e.g. software running on a processor,which requests the user to type numerical values of marker values andslopes, or which allows the user to draw the mathematical cost function602 graphically. The fixed audio reproduction strategy is e.g. the oneillustrated in FIG. 7. Here the reproduction quality R is embodied aswhat is called in the rest of the text a position reproduction qualityP, which is any specification of a physical parameter of the outputaudio signal o resulting in the perception that the output audio signalcomes from a position of a virtual sound source. E.g. the virtual soundsource can be perceived close to a user's head or far away, or in FIG. 7it is an angle α of a virtual sound source around a user's head. If theuser 100 is running with the desired target speed, the mathematical costc is zero and the angle α is also zero degrees, i.e. the virtual soundsource is right in front of the user 100. If the runner runs too slow orfast, the mathematical cost c with a specification like in FIG. 6 bdecreases resp. increases, and the virtual sound source shifts to theleft resp. right side of the user 100. The sound source can stay behindthe user until the user 100 runs with the desired speed of the chosenvalue d again or runs with the desired speed again for a certain amountof time. Alternatively, the sound source can also start behind the user100, being an annoying motivation to run harder.

In more advanced variants of the audio reproduction apparatus 200, theuser 100 can also specify the strategy for changing the reproductionquality as a function of the mathematical cost c. He can program a firstquality calculation unit 330 in FIG. 3 which outputs e.g. as a stereoquality S an angle 160 between a first virtual sound source 152 and asecond virtual sound source 154 generated by the output audio signal oand a second output audio signal o2, as a linear function of themathematical cost c. Or he can select an alternative or additional audioprocessing function, which e.g. adds an amount of reverberation as afunction of the mathematical cost c to simulate a distance of a virtualloudspeaker in a virtual room.

Another example application of the audio reproduction apparatus 200 isthe prevention of repetitive strain injury (RSI) or inactivity. In thiscase user 100 sits e.g. in front of a personal computer (PC) or on acouch in front of a television (TV). The sound production device 102 ise.g. a loudspeaker connected to the PC or television. The chosen value dis the amount of time the user 100 wants to work or watch TV beforetaking a break. The mathematical cost c is e.g. determined by the amountof time t elapsed since a starting time t0 minus the chosen value d,being the allowable time to work or watch TV continuously:c=(t−t ₀)−d if t−t ₀ >d; c=0 if t−t ₀ <d  [1]

With two loudspeakers of a TV or PC, a virtual sound source position canbe simulated.

FIG. 2 schematically shows an embodiment of the audio reproductionapparatus 200 in its basic form. An input audio signal i comes in via aninput means 204 from e.g. a portable MP3 player or the sound card of aPC. The input audio signal i can come form outside or inside the audioreproduction apparatus 200. In the latter case the audio reproductionapparatus 200 may comprise e.g. a CD player unit or any other internalaudio source 201. The input audio signal i can be mono or multichannelaudio. There is also a cost input 208 for receiving a mathematical costc from a mathematical cost calculation unit 210, which is arranged toderive the mathematical cost from a measurement made by a measurementdevice 212. The mathematical cost calculation unit 210 can beincorporated in the audio reproduction apparatus 200 or can be separate,e.g. in the measurement device 212. The measurement device 212 istypically not incorporated in the audio reproduction apparatus 200,although it could be, in case it is e.g. a clock. The audio reproductionapparatus 200 contains a conditioning unit 202, which contains an audioprocessing means 216 arranged to process the input audio signal i toderive the output audio signal o with a reproduction quality R independence of the mathematical cost c. The output audio signal o goes toan output 206, to which a loudspeaker 214 can be connected. The audioprocessing means 216 can just perform a single parametric functionleading to an output audio signal o of variable reproduction quality andhence perceptible psycho-acoustical quality, or multiple audioprocessing algorithms can be applied alternatively or simultaneously asin FIG. 3.

FIG. 3 schematically shows an audio processing means 316, being anembodiment of the audio processing means 216 of the audio reproductionapparatus 200. In the audio processing means 316 a number of processingunits are shown purely for explaining various features of the audioreproduction apparatus 200, and it should be clear that othercombinations are possible. The audio processing means 316 is arranged tosupply an output audio signal o to a first loudspeaker 314 and ifrequired a second output audio signal o2 to a second loudspeaker 315.

For many audio processing algorithms, the reproduction quality R can beset in advance, and a subsequent audio processing is chosen depending onthe reproduction quality R. E.g. the reproduction quality R can be aparameter of an audio processing algorithm, as in the case where theamplitude of the output audio signal is set. This can be realized with avariable gain amplifier. In other cases, user panel tests or thepreferences of the actual user of the audio reproduction apparatus 200can be used to select an appropriate processing algorithm, e.g. thefirst, second or third processing algorithm, 320, 322 resp. 324. In theexample embodiment of FIG. 3, a third mathematical cost c3 from a secondmeasurement device 352 goes to an algorithm selector 326, which e.g.contains a table of intervals. If the third mathematical cost c3 fallswithin a first interval, the first processing algorithm 320 is selected,etc. Such a configuration makes it possible to switch to entirelydifferent algorithms dependent on the value of the third mathematicalcost c3. E.g., the first algorithm may change the angle 160 between twovirtual loudspeakers, depending on where in the first interval the thirdmathematical cost c3 falls. If the third mathematical cost c3 becomes sohigh that it falls outside the first interval and in a second interval,the second processing algorithm 322 is selected, which e.g. changes theamplitude of the signals from the virtual loudspeakers, or both theangle 160 between them and the signal amplitudes. Another example inwhich the reproduction quality R is set prior to the processing is thesetting of an angular position on a sphere around the user's 100 head ofa virtual sound source by means of a head related transfer functionHRTF. E.g., when the user 100 wears headphones, the input audio signal ican be simulated to come from a virtual sound source position, byfiltering it by means of filter 332 using a specific first HRTF toobtain the output audio signal o for the left headphone loudspeaker 114and using a specific second HRTF to obtain the second output audiosignal o2 for the right headphone loudspeaker 115. Both HRTFs aredependent on the required position of the virtual sound source—e.g.specified as two angles on a unit sphere—and can be fetched from amemory 334 containing HRTFs for a number of different positions. A firstquality calculation unit 330 determines the reproduction quality Rneeded for further audio processing. E.g. in the above described casethe first quality calculation unit 330 calculates an angle a of thevirtual sound source, used for fetching the HRTFs, as a linear functionof a first mathematical cost c1. The first mathematical cost c1 isderived from a measurement device 312 by a mathematical cost calculationunit 310, which e.g. evaluates a function like the one in FIG. 6 a.Details on measuring HRTFs can be found in patent WO 01/49066 and paper“F. L. Wightman and D. J. Kistler: Headphone simulation of free fieldlistening. I: Stimulus synthesis. Journal of the Acoustical Soc. ofAmerica 85 no. 2, February 1989, pp. 858-867”.

In other cases, the reproduction quality R has to be measured on theoutput audio signal o itself, e.g. because the relation between thereproduction quality R of the output audio signal o and the particularprocessing is too complex to formulate or unknown. In this case feedbackcan be used to select the right processing algorithm or the rightparameter for a parametric processing algorithm. Quality measuring means344 measure an output quality measure M of the output audio signal o.The output quality measure M and a desired reproduction quality R from asecond quality calculation unit 340 are fed to a parameter valuecalculation means 346. From these two parameters, a parameter value pvis calculated for steering subsequent processing by an audio processingunit, which selects a particular processing algorithm or changes aparameter of a parametric algorithm. This can be done by any techniqueknown from control theory. E.g. An update parameter value pv can becalculated as in equation [2]:pv=δ(M−R)  [2]

Actually parameter value pv can be any function of M and R, if necessaryalso taking into account that the output quality measure M is adifferent function of the desired psycho-acoustical quality than thereproduction quality R.

The required implicit functionality between the output quality measure Mand the reproduction quality R derived from the first mathematical costc1, can be specified by user 100. With user input means 360, e.g. a keyboard, a touch sensitive panel, or a turning knob and user interfacemeans 311, the user 100 can specify a number of desired measurementvalues d, which are converted to corresponding first mathematical costsc1 and reproduction qualities R. Instead of inputting desiredmeasurement values d, the user 100 can also input desired mathematicalcosts cs. During this learning stage, for each reproduction quality R anumber of processing algorithms with corresponding output qualitymeasures M is scanned. When the output quality measure M correspondingto a psycho-acoustical quality as desired by the user 100 is reached,the user 100 can indicate this to the parameter value calculation means346 via a learning control connection 1 c—wired or wireless. Theparameter value calculation means 346 can then store the pairreproduction quality R and parameter value pv, so that during operation,the feedback is no longer needed, but rather that from a reproductionquality R corresponding to a measurement m, the correct parameter valuepv can be sent to an audio processing unit 342. The leaning controlconnection 1 c can also be used to specify which of the availableprocessing should be used for obtaining an output audio signal o withthe desired reproduction quality R, by setting an output selector 370.

Another example of putting user preferences in the audio processingmeans 316 is illustrated with a second user input means 361. In thisexample, the user 100 can directly enter a second desired mathematicalcost cs′ and a corresponding desired processing algorithm selection nainto the algorithm selector 326.

Note that in many cases the exact perceived psycho-acoustical quality,e.g. the exact position of a virtual sound source is not important, butonly that the psycho-acoustical quality changes monotonically. Thisrelaxes the requirements on the reproduction strategy. Any mapping ofvirtual sound source angle to mathematical cost e.g. might already besufficient.

Many processing algorithms can be designed to create some perceptiblepsycho-acoustical quality corresponding to a reproduction quality Rcharacterizing a selected algorithm. E.g. a reproduction of bassfrequencies can be changed in dependence of the mathematical cost. Asshown in FIG. 8, as a particular reproduction quality R or part of areproduction quality R, a specification SPEC can be calculatedreflecting the spectral content of the output audio signal o. Oneexample of the specification SPEC is a frequency below which there issubstantially no sound energy present, e.g. a first low frequency FL1 ora second low frequency FL2. E.g. if the user runs at nearly the desiredspeed, bass frequencies are reproduced all the way down to the first lowfrequency FL1. However if he runs to slow, he looses the bassfrequencies between the first low frequency FL1 and the second lowfrequency FL2. Another example of the specification SPEC is thepercentage of energy in bass range [FL1, FL2] compared to the energy inrange [FL3, FH]. Any equalization strategy can be employed as a functionof the cost c, e.g. the amount of trebble may be a function of the costc.

An interesting algorithm sets the mathematical cost function by means ofa three-dimensional bubble 150 around the user's 100 head. If he runstoo slow, the distance between the virtual position of his head 158 anda mark point 156 in the bubble increases, which leads to an increasedmathematical cost c, and a decreased reproduction quality R. Thereproduction quality R can also change in dependence of whether the user100 is inside the bubble 150 or not, which gives him a trainingtolerance. The virtual movement of the bubble 150 compared to therunning of the user could even keep track of whether the user waswaiting for traffic lights, this situation being identified e.g. when hepushes a button. When inside the bubble 150, the sound could e.g. soundas if the user 100 is in a particular room, by selecting HRTFscorresponding to that room, whereas outside the bubble 150, the soundsounds dull.

The distance of a virtual sound source can also be simulated. Theaudio-processing unit 342 can e.g. simulate the distance of the virtualsound source by changing the amplitude of the output audio signal o andsecond output audio signal o2. Or reverberation can be added. In a roomif a sound source is nearby, there is little reverberation, whereas ifthe sound source is far, there is a greater proportion of reverberation.Additionally or alternatively, virtual corridors can be generated, onwhich the virtual sound source reflects its sound.

A number of options are also possible for changing a stereo quality S ofa stereo audio signal. E.g. an angle 160 between a first virtual soundsource 152 and a second virtual sound source 154 can be diminished asthe mathematical cost c goes up, resulting in a lower stereo quality S.Or the first and second output audio signal o resp. o2 can be made moresimilar. Or there can be a hard switch from stereo to mono. Under stereosignal should also be understood multichannel audio, and a correspondingstereo quality S is e.g. the number of channels.

An example of a gradual change between stereo and mono is realized bycalculating changed stereo signals L′ and R′ according to the followingequations:L′=(1+a)L/2+(1−a)R/2R′=(1−a)L/2−(1+a)R/2  [3]

By changing the value of a parameter a between 0 and 1, a change betweenmono and stereo is effected.

The measurement device 312 and second measurement device 352 can beanything, e.g. a clock or a G.P.S. sensor indicating the position of theuser 100. In particular it could be a biometric measurement device, suchas a pace meter connected to a running shoe, a chest strap or ear-lobeheart rate meter 106, a thermometer, etc. These measuring devices areinteresting when used for sports performance measurement. Themeasurement device 312 could also be incorporated in a professionaltraining apparatus, e.g. a rolling belt for indoors jogging. The styleof running—particularly if unhealthy—could also be fed back.

In a game application, a second mathematical cost c2 can be set by acost determining means 313, e.g. a random generator. Depending on hisluck, the user's 100 mathematical cost is set back to the secondmathematical cost c2 and he has to run harder to re-achieve the level ofthe third mathematical cost c3, as determined by a second mathematicalcost calculation unit 350 from the second measurement device 352.

FIG. 4 schematically shows an embodiment of the audio feedback system.An audio source 421 delivers an input audio signal i to a conditioningunit 402. The audio source 421 can be e.g. a portable audio player or anaudio distribution server in a fitness center. A measurement device 412performs a measurement m, which is converted by a mathematical costcalculation unit 410 to a mathematical cost c, which is also input inthe conditioning unit 402. The conditioning unit 402 contains an audioprocessing means 416, which are arranged to process the input audiosignal to obtain an output signal o of a reproduction quality Rdependent on the mathematical cost c, which is sent to a soundproduction means 414, e.g. a pair of headphones or the loudspeakers of atelevision. The measurement device 412, mathematical cost calculationunit 410, conditioning unit 402, audio source 421, and sound productionmeans 414 can be realized separately or in any combination. E.g.,typically an audio reproduction apparatus may contain the conditioningunit 402, audio source 421 and mathematical cost calculation unit 410.

A head tracker may be present, so that the positions of virtual soundsources are corrected for movements of the user's 100 head. Also meansmay be present —e.g. a microphone—to pick up certain sounds from theenvironment and mix them with the signal for the sound production device414 for improved safety.

Specification of e.g. the cost function may instead of being done by theuser 100 come over a channel—such as e.g. internet—through an interface,from a second person, e.g. a personal trainer. Or the specification maybe done at a different time by the user 100, e.g. on his PC. E.g., hecan make a training schedule for the whole month, which can bedownloaded e.g. wirelessly to the audio reproduction apparatus.Parameters of functions and functions may even be downloaded from e.g.internet, and e.g. shared between users who want similar trainingsessions. Specifications made during training and stored in a memory,may also be downloaded through the interface to the computer for furtheranalysis, e.g. training improvement.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention and that those skilled in the art areable to design alternatives, without departing from the scope of theclaims. Apart from combinations of elements of the invention as combinedin the claims, other combinations of the elements within the scope ofthe invention as perceived by one skilled in the art are covered by theinvention. Any combination of elements can be realized in a singlededicated element. Any reference sign between parentheses in the claimis not intended for limiting the claim. The word “comprising” does notexclude the presence of elements or aspects not listed in a claim. Theword “a” or “an” preceding an element does not exclude the presence of aplurality of such elements.

The invention can be implemented by means of hardware or by means ofsoftware running on a computer, and previously stored on a data carrieror transmitted over a signal transmission system.

1. An audio reproduction apparatus comprising: input means for inputtingan input audio signal; an output for outputting an output audio signalderived from the input audio signal; a cost input for inputting amathematical cost derived from a measurement, which measurement isuser-influenceable; and a conditioning unit, capable of delivering theoutput audio signal in dependence of the mathematical cost,characterized in that the conditioning unit comprises an audioprocessing means arranged to process the input audio signal to derivethe output audio signal with a reproduction quality in dependence of themathematical cost.
 2. An audio reproduction apparatus as claimed inclaim 1, wherein the reproduction quality comprises a three-dimensionalposition of a virtual sound source, the audio processing means beingable to simulate the virtual sound source by means of the output audiosignal.
 3. An audio reproduction apparatus as claimed in claim 2,wherein the audio processing means comprises a filter arranged tosimulate the position of the virtual sound source by deriving the outputaudio signal by filtering the input audio signal with a user dependenthead related transfer function.
 4. An audio reproduction apparatus asclaimed in claim 2, wherein the audio processing means comprises anaudio processing unit arranged to simulate the position of the virtualsound source by changing a property of the output audio signal selectedfrom signal amplitude and added reverberation.
 5. An audio reproductionapparatus as claimed in claim 1, wherein the audio processing means isarranged to derive a second output audio signal, together with theoutput audio signal constituting a stereo audio signal, the audioprocessing means being arranged to derive the stereo audio signal fromthe input audio signal with a specified stereo quality dependent on themathematical cost.
 6. An audio reproduction apparatus as claimed inclaim 1, wherein the reproduction quality comprises a specification of adistribution of frequencies of the output audio signal.
 7. An audioreproduction apparatus as claimed in claim 1, comprising a first qualitycalculation unit for determining the reproduction quality for use in thesubsequent derivation of the output audio signal by the audio processingmeans.
 8. An audio reproduction apparatus as claimed in claim 1,comprising quality measuring means for measuring an output qualitymeasure of the output audio signal, and comprising parameter valuecalculation means for calculating a parameter value, for use in thesubsequent derivation of the output audio signal by the audio processingmeans.
 9. An audio reproduction apparatus as claimed in claim 1, whereina mathematical cost calculation unit is comprised which is arranged toderive the mathematical cost from the measurement receivable from ameasurement device.
 10. An audio reproduction apparatus as claimed inclaim 9, wherein the mathematical cost calculation unit is arranged toderive the mathematical cost based on a difference between themeasurement and a chosen value.
 11. An audio reproduction apparatus asclaimed in claim 9, wherein the mathematical cost calculation unit isarranged to derive the mathematical cost from a biometric measurement.12. An audio feedback system comprising: an audio source; a measurementdevice arranged to deliver a measurement which is user-influenceable; amathematical cost calculation unit, arranged to derive a mathematicalcost from the measurement; a sound production device; and a conditioningunit arranged to receive an input audio signal from the audio source, toreceive the mathematical cost, and to deliver to the sound productiondevice an output audio signal derived from the input audio signal, independence of the mathematical cost, characterized in that theconditioning unit comprises an audio processing means arranged toprocess the input audio signal to derive the output audio signal with areproduction quality in dependence of the mathematical cost.
 13. Amethod of deriving an output audio signal from an input audio signal independence of a mathematical cost derived from a measurement which isuser-influenceable, characterized in that the output signal is derivedwith a specified reproduction quality dependent on the mathematicalcost.
 14. A computer program for execution by a processor, describingthe method of claim
 13. 15. A data carrier storing the computer programof claim 14.